Public procurement and sticking to single market rules
The Continent's common market has been the cornerstone of European integration for the past half century. If it hadn't been created, or if Ireland hadn't become a part of it, there is reason to believe that we would still be the poorest country in western Europe - as we were for most of the period from independence to the 1990s.
Removing barriers to cross-border economic activity has taken place on the foundation of the 'four freedoms' - the free movement of capital, labour, goods and services. Implementation from Strasbourg and Brussels has happened gradually, and grindingly, over decades.
Even after tariffs and quotas were formally abolished in 1968, plenty of barriers to cross-border trade remained. The costs of operating in different countries stayed high. Each member state had a different set of laws, national standards and regulations. Either by accident or design, governments continued to put in place barriers which impeded trade and, more often than not, tended to favour domestic firms.